Cytolytic lymphocytes (CTLs), which include cytotoxic T cells and natural killer cells, can recognize and eliminate a wide variety of virus-infected or transformed target cells. The molecular mechanisms used by these cells to induce target cell death are incompletely understood. A large body of experimental evidence supports the granule exocytosis model, in which target cell recognition results in the release from the CTL of dense-cored cytoplasmic vesicles containing putative effector molecules such as perforin and the serine proteases (Martz et al., Immunol. Today 10:79-86, 1989; Tschopp and Nabholz, Annu. Rev. Immunol. 8:279, 1990; Young and Liu, Immunol. Today 9:140-144, 1988). Perforin has been shown to be directly cytolytic (Hameed et al., J. Exp. Med. 169:765-777, 1989; Lichtenheld et al., Nature 335:448-451, 1988; Shiver and Henkart, Cell 64:1175-1181, 1991). After inserting into target cell membranes, it polymerizes to form non-specific ion channels through which markers of intracellular compartments can readily pass (Tschopp et al., Nature 337:272-274, 1989; Young et al., Proc. Natl. Acad. Sci. 83:150-154, 1986; Yue et al., Mol. Immunol. 24:647-653, 1987). The formation of these ion channels appears to be sufficient to induce the lysis of certain cell types. Although purified serine proteases are not directly cytotoxic, the ability of protease inhibitors to block lymphocyte-mediated cytolysis suggests that these granule components might also play a role in target cell killing (Lavie et al., J. Immunol. 135:1470-1476, 1985; Rodgers et al., J. Immunol. 140:564-570, 1988).
In addition to perforin-mediated cytolysis, considerable evidence suggests that target cell death can also result from the induction of an endogenous pathway of programmed cell death. Central to this autolytic pathway is the activation of an endogenous endonuclease that results in the degradation of target cell DNA into integer multiples of a 200 bp nucleosome-sized monomer (Duke et al., Proc. Natl. Acad. Sci. USA 80:6361-6365, 1983; Wyllie, Nature 284:555-556, 1980). The resulting "ladder" of DNA fragments is considered to be characteristic of this programmed suicide pathway. Isolated CTL granules have been shown to induce both cell lysis (measured by the release of .sup.51 Cr) and DNA fragmentation (measured by the appearance of nucleosome-sized DNA fragments) in target cells (Allbritton et al., J. Exp. Med. 167:514-527, 1988; Podack and Konigsberg, J. Exp. Med. 160:695-710, 1984). However, studies which used purified perforin have shown that it, while capable of inducing cell lysis, does not induce DNA fragmentation in target cells (Duke et al., J. Exp. Med. 170:1451-1456, 1989).